Predatory Behavior and Neuroanatomy of the Sessile Rotifer, Cupelopagis vorax

Abstract

Predator-prey interactions contribute to the evolution of behavioral and morphological characteristics of species. However, research on zooplankton has mainly focused on planktonic species such Mesocyclops edax (Arthropoda, Cyclopoida) and Asplanchna spp. (Rotifera, Ploima). Cupelopagis vorax (Rotifera, Collothecaceae) provides a unique model for studying the predatory behavior of a sessile species. Cupelopagis was chosen because it is the only rotifer known to exhibit rheotaxic behaviors in the presence of prey and because it undergoes indirect development where non-feeding, free-swimming larvae mature into feeding adults. The integration of behavioral techniques, immunohistochemistry, and confocal microscopy were used to provide insight as to how C. vorax feeds. Single prey and mixed prey feeding experiments were conducted using two prey species, the rotifer Lepadella triba and the gastrotrich Lepidodermella squamata. The predatory behavior of Cupelopagis was classified into three categories: encounter, attack, and capture. When predator-prey interactions for both species were compared, there was no significant difference in the frequency of encounters (W = 80.5, p = 0.19). However, attacks were more frequent for gastrotrich prey (W = 44, p < 0.01) while capture events were higher for rotifer prey W = 189.5, p = 0.01). Ivlev's electivity index (Ei) and Manly-Chesson's index (αi) were used to assess prey selectivity. Only mean αi values indicated active selection of gastrotrich prey by C. vorax; however, more evidence is needed to confirm if Cupelopagis is a selective feeder. The feeding behavior of Cupelopagis is influenced by its ability to respond to its environment. Overall, these results provide additional evidence that Cupelopagis uses mechanoreceptors to detect potential prey based on its reaction (attack and capture) when prey were within proximity of the infundibulum. It is also proposed that chemoreceptors may aid in the ingestion and rejection of prey given its tendency to prefer gastrotrich prey. To gain insight into the neural basis of these behaviors, fluorescent labeling was employed to describe serotonin immunoreactivity (5HT-IR) in Cupelopagis larvae and adults. The brain was dorsal to the mastax and bilaterally symmetrical. In larvae, paired lateral nerves innervated the corona, mastax, and foot. In adults they innervated the infundibulum and proventriculus. Both larvae and adults had four 5HT-IR perikarya in the central nervous system. However, metamorphosis had an effect on the structure and position of 5HT-IR neurons in C. vorax. Larvae contained a single pair of anterior and posterior perikarya in the brain that were arranged in an X-shaped pattern. While in adults, the arc-shaped brain ganglion contained four single posterior perikarya. Perikarya in larvae were generally larger (1.7X) than those found in adults but their function was not discerned. In total, these results seem to indicate that the serotonergic nervous system of C. vorax is structured to meet the metabolic and physiological requirements of each developmental stage (i.e. locomotion in larvae and feeding in adults). However, more research is needed to elucidate function from structure. The results of this study can help define the behavioral and sensory mechanisms involved in the evolution of feeding behavior within phylum Rotifera. It also increases our understanding of how the nervous system evolves and develops in invertebrate organisms.